docs: def _get_collection(self, collection_name):
          """
          Get PyMongo client pointing to the current DB and the given collection.
  
          :return: MongoDB client of the current database and given collection.
          :raise DataSourceError
          """
          try:
              return self._database[collection_name]
          except InvalidName as ex:
              raise DataSourceError("Cannot access MongoDB collection %s!"
                                    % collection_name) from ex
          except Exception as ex:
              raise DataSourceError("Unexpected error when accessing MongoDB"
                                    "collection %s!"
                                    % collection_name) from ex